Field of the Invention
The present invention relates to a narrow-angle directional microphone that allows an acoustic tube to be atachably/detachably mounted to a main body case that includes a microphone unit, and especially relates to a narrow-angle directional microphone with a devised attaching structure of the acoustic tube to the main body case.
Description of the Related Art
As a narrow-angle directional microphone, one using a long and narrow acoustic tube, and having a microphone unit arranged near a rear end portion of the acoustic tube in a length direction is known. This narrow-angle directional microphone has a characteristic to detect only a sound wave entering from a direction of a front end portion of the acoustic tube.
In the acoustic tube, the front end portion thereof is open, a slit-like sound wave introducing port is formed in a side wall, and an acoustic resistance material is affixed to the sound wave introducing port. Accordingly, regarding the sound waves from directions except the direction of the front end of the acoustic tube, the sound wave entering from the front end of the acoustic tube, and the sound wave entering through the sound wave introducing portion in the side wall of the acoustic tube have a time difference, thereby to interfere with each other and cancel each other.
Such a microphone is also called shotgun microphone, and is a narrow-angle directional microphone having high sensitivity to the sound wave from the direction of the front end portion of the acoustic tube.
According to the narrow-angle directional microphone including the acoustic tube, characteristics of narrow-angle directivity depend on the wavelength of the sound wave and the length of the acoustic tube. That is, when a long acoustic tube is used, the narrow-angle directivity can be obtained with frequencies up to a low frequency, and when a short acoustic tube is used, the narrow-angle directivity can be obtained only with a high frequency. To use the difference of the characteristics of depending on the length of the acoustic tube, a microphone that allows an acoustic tube to be attachable/detachable to/from a main body case in which a microphone unit is arranged, that is, a narrow-angle directional microphone that allows acoustic tubes having different axial lengths to be replaceable, as needed, has been proposed.
Conventionally, as means for attaching the acoustic tube to the main body case, means for attaching the acoustic tube with fixing screws in three places in a peripheral direction, for example, in a joint portion of a front end portion of the main body case with the acoustic tube, is typically employed.
FIGS. 4A and 4B and FIG. 5 of JP 2012-227600 A (hereinafter, called Patent Document 1) disclose the above-described means for attaching the acoustic tube with the fixing screws.
FIGS. 4A and 4B of the above-described application illustrate an example of attaching the acoustic tube to the main body case side using the fixing screws disclosed in Patent Document 1. Note that the example of FIGS. 4A and 4B illustrates a principal portion of a configuration in which the acoustic tube is attached to the main body case through a relay member mounted to the main body case, by enlarged sectional views.
The relay member 12 has a rear end portion to which a cylindrical main body case 11 is attached, and a front end portion side of the relay member 12 is reduced in an outer diameter, and a tip end portion of the front end portion side supports the microphone unit 1.
As illustrated in FIG. 4A, the microphone unit 1 includes a diaphragm 3 at a front end portion in a unit case 2, and a fixed electrode 4 facing a back surface of the diaphragm 3. Further, an electrode drawing part 5 is arranged on a back surface of the fixed electrode 4. An air chamber 6 communicating into the back surface of the diaphragm 3 is formed between the electrode drawing part 5 and the fixed electrode 4.
A sound hole 5a communicating into the air chamber 6 penetrates the electrode drawing part 5 in an axial direction and formed therein. An acoustic resistance body 7 formed in a doughnut shape is arranged on a back surface of the electrode drawing part 5, and blocks the sound hole 5a. 
The acoustic resistance body 7 is sandwiched between an adjusting ring 8 arranged on a back surface of the acoustic resistance body 7 and the electrode drawing part 5, and the degree of compression of the acoustic resistance body 7 is adjustable. Accordingly, the acoustic resistance by the acoustic resistance body 7 is configured to be variable. Then, a sealing member 9 that blocks the back of the unit case 2 is attached to a back surface of the adjusting ring 8 with a small gap. Further, a base member 10 that supports the unit case 2 is attached to a rear end portion of the unit case 2, and an axial hole formed in a central portion of the base member 10 is fit into the tip end portion of the relay member 12. Then, a microphone unit 1 is held in the front end portion of the relay member 12.
An insulating base 13 is inserted into and arranged in the relay member 12. A drawing rod 14 that comes in contact with the electrode drawing part 5 is embedded in a central portion of the insulating base 13.
Although not illustrated, a circuit board on which an impedance conversion circuit and the like are mounted is arranged in the main body case 11 attached to the rear end portion of the relay member 12. Further, an output connector into which a microphone output is led is arranged in a rear end portion of the main body case 11, the microphone output having been signal-processed in the circuit board.
A ring-like step portion 12a is formed in the relay member 12. The external shape around a rear end portion of the step portion 12a is made slightly narrow. A rear end portion of a cylindrical acoustic tube 21 is attached to abut against the step portion 12a. Further, a sound conduction hole 21a is formed in a side wall of the acoustic tube 21, the side wall being adjacent to the unit case 2. A sound conduction hole 2a formed in the side wall of the unit case 2 communicates into an outside through the sound conduction hole 21a. 
Therefore, the sound conduction hole 21a formed in the acoustic tube 21 functions as an acoustic taking-in hole of the microphone unit 1 at a rear portion side. Accordingly, the microphone unit 1 configures a unidirectional condenser microphone.
Then, a rear end portion of the acoustic tube 21 is attached to the relay member 12 with fixing screws 23 screwed into the relay member 12 at three places in the peripheral direction. Accordingly, the acoustic tube 21 and the main body case 11 are coaxially connected through the relay member 12 in a front and rear direction.
By the way, the entire length of the narrow-angle directional microphone is long. Therefore, the narrow-angle directional microphone has a problem of being easily damaged when stress is applied to the relay member 12 that is a joint portion of the acoustic tube. Especially, as illustrated in FIG. 4B, in a case where the acoustic tube 21 is attached to the relay member 12 using the fixing screws 23, and when bending stress acts on the acoustic tube 21, a screw insertion hole 21b at the acoustic tube 21 side is deformed. Accordingly, a problem that a gap is caused between the screw insertion hole 21b and the fixing screw 23 in an axial direction, and the fixing screw 23 is loosened is caused. The looseness of the fixing screw 23 caused as described above becomes a cause of occurrence of noises during the use of the microphone.
Especially, in this sort of microphones, a microphone unit with a large effective area needs to be mounted in order to improve the sensitivity and a signal-to-noise ratio. Further, the thickness of a tube wall of the acoustic tube 21 is required to become as thin as possible in order to decrease the weight of the entire microphone. Due to the above reasons, mechanical strength of the acoustic tube 21 is further decreased, and the above-described problem becomes more prominent.
Note that the configuration of the microphone 1 illustrated in FIG. 4B is the same as the one illustrated in FIG. 4A. Therefore, description of the reference numerals of the respective units that configure the microphone unit 1 is omitted in FIG. 4B.